Hydrostatic transmissions have many advantages and have found ever increasing application in recent years. However, due to the poor performance of its major components (especially the hydraulic motor or pump), its application has been very limited.
The application of hydrostatic transmissions in vehicle drive systems has numerous advantages, such as: a genuine continuously variable transmission within its full speed range; equal speeds in forward and reverse; very smooth speed change; the best matching between the engine and the transmission to improve its fuel economy and dynamic performance; easier adaptation to automatic control; and its convenient layout in the vehicle.
Hydrostatic transmissions of wheel motor type are an ideal power transmission system for most vehicles. These transmissions consist of a pump driven by an engine, the high pressure oil from the pump being delivered to hydraulic wheel motors in the wheels via control valves, hoses or pipes to generate a driving torque to propel the vehicle. Obviously, this layout could take full advantage of the merits of hydrostatic transmissions and offer many other advantages in addition to those above mentioned. These include the following:
(1) The construction of the transmission system can be significantly simplified, especially for those vehicles requiring large speed ratio range of multiple-speed steps or required to execute complicated operations.
(2) The layout of the vehicle can be simplified because the engine is connected to the wheel motor by hoses or pipes, without regard to their relative positions which is valuable in many vehicles, such as, minicar or ultra-minicar, self-propelled agricultural machines, military vehicles, construction machinery, etc.
(3) Easy realization of interchangeability and series design of the parts and components of the transmission system, so the requirements of different vehicles can be met by properly combining a series of components.
Since the hydrostatic transmission of wheel motor type has so many attractive advantages, it has often been the subject of designers and researchers. However, limited by the performance of its components (especially the wheel motors), the hydrostatic transmission of wheel motor type has, at present, only been used in some low speed or special purpose vehicles. Therefore, in order to adapt this technology effectively to middle or high speed vehicles, performance and construction of the wheel motors must be improved in order to satisfy the following requirements:
(1) It must be directly mounted in the wheels to drive the wheels without any additional speed reducing/increasing gearboxes.
(2) The hydromotor should have a rather wide speed range to satisfy the speed requirements of the vehicle. For example, for a car it must operate normally in the speed range of 0 to 1000 r.p.m. or above, and its efficiency must not decrease significantly with increased speed. The maximum speed of today's wheel motors is about 200-300 r.p.m., falling short of the above requirement, so its application is limited to low speed (less than 20-30 kilometer per hour) vehicles.
(3) The hydromotor should have high efficiency over a wide speed range. Most vehicles require high tractive forces during starting and climbing, which may be 20-30 times the relatively light load in normal operation. For instance, transportation trucks are running, for the most part, under light loads at medium to high speeds, just within the low efficiency region of today's hydraulic wheel motors. But fuel economy (overall efficiency) is often the major consideration in this kind of vehicle, which results in very strict demands on the efficiency performance of wheel motors. Specifically, it must have very high torque efficiency in starting as well as overall efficiency under light loads over a wide speed range. These requirements have not been met by the present hydromotors.
(4) The displacement of the hydraulic wheel motor should be variable to meet the requirement of further extending the speed range of the vehicle. Vehicle speed and load varies over a wide range in its operation and the maximum tractive effort required. Because of this, in a fixed displacement motor, the working pressure may fall to 10-30 kg/cm.sup.2 during normal running, far from its high efficiency region. Moreover, at high vehicle speeds, the oil flow velocity in the hydraulic system increases proportionally with the vehicle speed resulting in a high flow loss, further reducing the overall efficiency of the system. If the displacement of the hydraulic motor is made variable, then its displacement can be decreased at above-mentioned working conditions to increase the working pressure and reduce the velocity of the oil flow to improve the overall efficiency of the system.
(5) It is necessary to simplify the construction and to lower the requirement on material and manufacturing technology so that hydrostatic transmissions may be more adaptable for mass production to cut down the cost and price. At present,the cost of hydrostatic transmissions is often much higher than mechanical transmissions, which is one of the main factors limiting its application.
(6) The hydraulic wheel motor must operate under the adverse conditions which may be encountered in its use, for instance, it should safely withstand the vibration and shock loads in operation, not be too sensitive to the working fluid and its filtration, easily repaired and maintained, etc.
To solve the problems mentioned above, attempts have been made to improve the construction of hydraulic wheel motors and vehicle hydraulic transmission systems. As an example,
S.A.E. paper No. 790883 "An Interesting And Informative Comparison Of Mobile Hydrostatic Wheel Hub Drives" and S.A.E. 810971, 810974 have systematically described the present state of this art and emphatically compared the performance of the two popular types of hydraulic wheel motors today.
One type, the axial piston hydraulic motor with a planetary gearbox, is widely used in some vehicles, exploiting fully its capability of high speed, high pressure, high efficiency and continuously variable displacement. However, this type is characterized by complicated construction, the high price of the axial piston hydraulic motor and planetary gearbox, narrow speed range, low overall efficiency (including the efficiency of the planetary gearbox) and starting torque efficiency and its irregular external shape limits its use in many other applications.
The second type, the cam lobe hydromotor, has a relatively high starting torque efficiency and mechanical efficiency as well as a wider speed range than the axial piston type. However, it is also complicated in construction with its efficiency reduced significantly with increasing speed and is incapable of continuously variable displacement. Therefore its application is also limited to some low speed (less than 20 kilometer per hour) or special purpose vehicles.
As can be seen from the above, the application of hydrostatic transmissions of the wheel motor type is dependent, to a large extent, on the perfection of its performance. There must be a breakthrough in construction and performance of the wheel motor before hydrostatic transmissions of the wheel motor type may enjoy wide application. Moreover, a more perfect hydraulic transmission system with further improved overall efficiency and extended speed range must be developed to exploit fully the merits of the wheel motor.